The present invention relates to a new and improved construction of phase breakdown- and null current-detector, especially for an electronic motor protection relay with current-dependent triggering, which is controlled by detection alternating-current voltages peoportional to the current values in the motor outer or external conductors and contains an electrical motor simulator for simulating the thermal behavior of the motor due to the charging operation of a capacitive storage, the motor simulator can be adjusted to different time-constants corresponding to the operating conditions of the motor by means of a controlled switching device.
In Swiss Pat. No. 541,885 there is taught to the art a circuit arrangement for the recognition or detection of defects or faults brought about by a phase breakdown or ground short in the current supply of a three-phase motor by means of detection alternating-current voltages proportional to current values in the motor outer conductors. There is provided a time-multiplex device and a rectifier circuit arrangement connected in series therewith in order to generate from the detection alternating-current voltages by means of cyclically successive samples a time-multiplex signal possessing signal amplitudes and signal frequency determined by the detection alternating-current voltages. For instance, from the positive portion of the time-multiplex signal there is formed a measurement voltage by smoothing or forming a mean value, the amplitude of which is approximately proportional to the peak value of the maximum detection alternating-current voltage or the arithemtic means value of the detection alternating-current voltages. For the purpose of phase breakdown-detection it is possible after differentiating the time-multiplex signal to compare in a comparator the voltage pulses possessing the maximum amplitude value with a percentual portion of the measurement voltage. By means of this comparison there is gradually detected and evaluated the asymmetry in the detection alternating-current voltages which arise during phase breakdown. For ground short detection there is derived from the time-multiplex signal by means of a low-pass filter for signal components occurring at the frequency of the detection alternating-current voltages the peak value which is compared in a second comparator with another percentual portion of the measurement voltage. Such circuit arrangements operate accurately and reliably, but are rather complicated.
Electronic protection relays for electrical operating means or equipment, such as transformers and especially motors, contain for the electrical simulation of the heating behavior thereof RC-elements which are supplied with a charging current which is dependent upon the operating current. The charging voltage of the capacitors is compared with a comparison voltage characteristic of the permissible boundary or threshold temperature of the operating means and, as soon as the charging voltage exceeds the comparison voltage, the protection relay responds and cuts-off the operating current. In order to be able to take into account the different heating behavior between an operating or running motor and a stationary motor, which is essential for any operation having intermittent standstill times, there has already been proposed to the art, for instance as taught in Swiss Pat. No. 540,587, to construct the simulator so that it can be switched to different time-constants, wherein the switching operation occurs by means of an electronic switching element, for instance a transistor, controlled by appropriate control signals. In this regard there can be obtained in part very large time-constants which, in the case of a direct-current supply of the simulator, only can be realized with extremely low charging currents and extremely high resistances. In such instances the charging current is supplied in a clocked or timed fashion to the simulator, and accordingly, there can be provided between the charging current source and the simulator an electronic switch controlled by a clock generator.
The clocking of the charging current for the simulator renders possible different advantageous possibilities for an electronic motor protection relay. Thus, for instance, there has already been proposed from Swiss Pat. No. 534,444 to make the clock frequency proportional to the measurement voltage in order to obtain a triggering or release characteristic corresponding to the protected object. The clock generator then contains, for instance, an integrator for the measurement voltage which in each instance is reset at a predetermined voltage value. Accordingly, electronic protection relays, depending upon requirements, can be relatively simple and inexpensive devices, but also rahter complicated and thus expensive precision devices.